Question - Why 4-20mA signal is prefered over a 0-10v?
Answer -
If the instrument is faulty, it can be detected easily. If the output is 0 ma, it will be hard to determine whether it is a defective signal or a good signal.
20 ma is used as maximum because the human heart can withstand up to 30 ma of current only. For safety reason 20 ma chosen as the maximum value.